Field of the Invention
The present invention is suitable for a color separation optical system used in an image pickup apparatus such as a television camera and a video camera, the color separation optical system including multiple prisms and dichroic films and configured to separate an entrance light into rays of multiple color lights and guide the separate rays respectively to image pickup elements.
Description of the Related Art
In a television camera and the like, a color separation optical system has been conventionally used as means for separation a ray exiting from an object lens into rays of multiple color lights and guiding the rays respectively to image pickup elements corresponding to the color lights. Many color separation optical systems each have, for example, three prisms and separated the entrance light into rays of three color lights of blue, green, and red.
For example, the color separation optical system has at least three prisms including: a first prism including a dichroic film reflecting the blue light and configured to extract the blue light; a second prism including a dichroic film reflecting the red light, disposed adjacent to the first prism with an air gap provided therebetween, and configured to extract the red light; and a third prism bonded to a surface (second reflection surface) of the second prism provided with the dichroic film, and configured to extract the green light transmitted through the first prism and the second prism.
Generally, a metal coating film is provided on a surface of each solid-state image pickup element used in the image pickup apparatus, and the reflectance thereof is high. In the color separation optical system, out of the color lights subjected to color separation, the color light color-separation in the second prism is reflected on the solid-state image pickup apparatus and thereby causes a ghost light in many cases.
There has been conventionally known a color separation optical system which reduces the ghost light caused by the second prism configured to color-separate the red light in the color separation optical system (U.S. Pat. No. 6,342,980, U.S. Pat. No. 8,310,765). Moreover, there has been known a color separation optical system which includes four prisms of a first prism to a fourth prism in this order from the object lens side and which achieves high resolving power by appropriately setting prism apex angles of the first prism and the second prism (U.S. Pat. No. 5,777,674).
In many image pickup apparatuses, solid-state image pickup elements are used as image pickup means. Accordingly, in the image pickup apparatuses using the solid-state image pickup elements, the ghost light and the interference fringe ghost are generated in many cases. In the color separation optical system described above, the ghost lights are generated by lights reflected on solid-state image pickup element surfaces as follows: the second color light reflected on the entrance surface of the solid-state image pickup element into which the second color light enters is reflected on the second reflection surface of the second prism and then reenters the solid-state image pickup element. In this case, the ghost and the interference fringe ghost are generated.
For example, the second color light color-separate by the second prism is sometimes reflected on the surface of the solid-state image pickup element to go back an optical path through which the light has come, and enters the second prism. Then, this light is reflected on the second reflection surface of the second prism to the inside of the second prism, is totally reflected on the second entrance surface, and thereafter reenters the solid-state image pickup element to generate the ghost light. In another case where total reflection condition is not satisfied in the second entrance surface, a ray is transmitted through the second entrance surface without being totally reflected, travels through the air gap, and is reflected on the first reflection surface of the first prism. Then this ray travels through the second prism and thereafter reenters the solid solid-state image pickup element to generate the interference fringe ghost.
U.S. Pat. No. 6,342,980 and U.S. Pat. No. 8,310,765 disclose the color separation optical systems configured to achieve reduction of the generation of the ghost and the interference fringe ghost. In these techniques, the prism apex angle of the second prism is made large. Accordingly, the sizes of the color separation optical systems tend to be large. In these circumstances, it is important to appropriately set the configuration of the color separation optical system in order to reduce the generation of the ghost and the interference fringe ghost caused by the reflection on the entrance surface of the solid-state image pickup element and obtain an excellent image while achieving size reduction of the color separation optical system.